Optical disk devices are used for the storage of computer prepared data and have recognized value in their ability to store large quantities of data. The media for use in such devices is reactive to bursts of light such as may be produced by the rapid switching of a semiconductor laser. In order to write data on optical media, the laser power must be controlled at a fairly high power level in order that the media can be altered to reflect the presence or absence of data. In reading the data back, the laser power level is controlled to a lower level so that the media is not altered by the laser beam.
Optical media is of two general types. That is, media which can be written only once and media which can be written, erased and written again. Write once media (WORM) is permanently altered when "write" power levels are produced by the laser beam while erasable media such as magnetooptic (MO) media is not permanently altered when data is written. In the MO media, the magnetic orientation of the reactive material is altered in the writing process and in the erasing process the magnetic orientation is reordered.
In operating an optical disk system it is necessary to set the correct laser power level to read and to write for each optical disk. The correct parameters for the optical disk are included in information in an identification header stamped onto the disk itself. That information, when read by the system, enables a calibration circuit to set the desired current levels for the laser to produce correct laser power. Since the laser is subject to unintended changes in its operating parameters particularly with temperature and aging, the calibration method is also used to change current levels for the laser so that the correct power level is maintained under operating conditions and throughout laser life.
The common practice of calibrating the laser circuits to operate with a given optical medium usually involves analyzing the laser light intensity at the optical medium. To do that, the laser control circuits are set to match a predetermined or desired light intensity at the optical medium. Analysis is conducted to enable setting digital to analog converters (DAC) for controlling laser power in the writing, and the erasing operations of optical mediums. With a WORM media, the high light intensities needed to calibrate the circuit might write on the disk and thereby cause permanent alteration of the media during the calibration process. Therefore, it has become common practice to calibrate WORM media with the laser beam out of focus at high power levels in order to calibrate without permanent alteration of the media. It is possible to use out of focus conditions since the optical path for WORM media is designed to eliminate light feedback reflected from the optical disk. With MO media, the common practice is to calibrate the system under in focus conditions in order to obtain accurate power levels since optical feedback cannot be eliminated from the optical path. One of the effects of such feedback is a change in the laser power versus bias current curve (P-I curve), and, therefore, calibration should take place under in focus conditions.
The inventors herein have sought to provide an optical disk apparatus capable of recording and reading both write once media and erasable media. In order to do that, the apparatus must provide a calibration technique which can be used on both types of media. For MO media, the power level for the "write" DAC calibration could be performed with the laser beam in focus. However, with WORM media, the "write" DAC power level cannot be calibrated with the laser beam in focus since permanent alteration of the media would result.
The above referenced patent application provides a digital to analog converter to control the output light intensity of a laser diode such as for recording signals on an optical disk. The laser power is calibrated by initially setting the DAC to supply two predetermined laser power levels, one a high level and one a low level. That is accomplished by increasing the numerical input to the DAC until beam intensity reaches values at predetermined laser power levels. A calculation in a microprocessor subtracts the lower power value from the higher power value, then divides the number (DAC setting) between the two power levels to obtain a DAC efficiency which is a laser output power level change per DAC unit value input change. The desired DAC settings for producing desired power levels are then determined. That calibration technique was designed for use with MO media and called for the application of power at a low level and at a high level in order to determine the linear slope of the P-I curve. Utilizing that technique with WORM media would destroy the media at the high power level.
U.S. Pat. No. 4,785,443 describes an optical disk system and laser light control circuit for WORM disk media. The "write" laser power calibration is performed with the media out of focus in order to protect the storage media from permanent alteration.